The objective of this study is to biochemically characterize retroviruses in order to understand the mechanisms by which these viruses induce cancer in their natural hosts. The role of these viruses in the etiology of human cancers is also under study. Studies currently in progress are the following: 1) biochemical characterization of replication-defective mammalian transforming viruses; 2) search for the presence of retroviral genes and gene products in human tumors.